Real-time display systems based on spatial light modulators (SLMs) are increasingly being used as an alternative to display systems using cathode ray tubes (CRTs). SLM systems provide high resolution displays without requiring the digital data to be converted to analog form prior to display as is the case with a CRT system.
Digital micro-mirror devices (DMDs) are a type of SLM, and may be used in projection display applications. A DMD has an array of micro-mechanical mirror elements, each mounted above a memory cell. Each mirror is individually addressable by electronic data. Depending on the state of its addressing signal, each mirror is tilted so that it either does or does not reflect light on or off an optical axis toward a display screen. The proportion of time during each video frame that a mirror is in an on state determines shades of gray--from full black for zero on-time to full white for 100 percent on-time.
One type of DMD display system is a projection system. Light from the "on" mirrors passes through a projection lens and creates images on a large screen. Light from the "off" mirrors is reflected away from the projection lens and trapped. Color may be added in either of two ways, by a color wheel or by a three-DMD configuration in which each DMD images a red, green, or blue light component onto the screen.
During operation, it is sometimes desirable to have completely black image. However, in existing systems, even when all of the DMD's mirror elements are in an off state, some light continues to reach the entrance aperture of the projection lens.